Ideally, a blade of the airfoil type is shaped like a typical aeroplane wing, where the chord width of the blade as well as the first derivative thereof increase continuously with decreasing distance from the hub. Ideally, the blade is comparatively wide in the vicinity of the hub, resulting in problems when having to mount the blade to the hub. Moreover, the large surface area of the blade near the hub causes large loads, such as storm loads, during use.
Therefore, over the years, the design of modern, wind turbine blades has developed towards a shape, where the blade consists of a root region closest to the hub, an airfoil area furthest away from the hub and a transition area between the root region and the airfoil area. The airfoil area has an ideal or almost ideal blade shape, whereas the root region has a substantially circular or elliptical cross-section, which reduces the extreme loads and makes it easier and safer to mount the blade to the hub. The root region diameter or chord is preferably substantially constant along the entire root region. Due to the circular or elliptical cross-section, the root region does not contribute to the power production of the wind turbine and, in fact, lowers the production because of drag. As is suggested by the name, the transition area has a shape gradually changing from the circular shape of the root region to the airfoil profile of the airfoil area. Typically, the width of the transition area increases substantially linearly with increasing distance from the hub.
As for instance blades for wind turbines have become bigger and bigger in the course of time and may now be more than 60 meters long, the demand for optimised aerodynamic performance has increased. The wind turbine blades are designed to have an operational lifetime of at least 20 years. Therefore, even small changes to the overall performance of the blade may over the lifetime of a wind turbine accumulate to a high increase in economical gains, which surpasses the additional manufacturing costs relating to such changes. The focus areas for research have in many years been directed towards improving the airfoil region of the blade, but during the recent few years, more and more focus has been directed towards improving the aerodynamic performance of the root region and the transition region of the blade also.
It is well known from the aeroplane industry that aeroplanes built with two wings, so called biplanes, normally can lift more than an aeroplane with only one wing. This allows for an increase of the total lift of the wings of the aeroplane without increasing the width of the wings. This principle of using two blades is also known in connection with blades for wind turbines, e.g. by manufacturing wind turbines with two or more rotors. As an example, JP56138465A2 discloses an auxiliary propeller for a wind turbine, where the auxiliary propeller is mounted on the same rotor as a main propeller. The auxiliary propeller has a larger pitch angle than the main propeller, such that the auxiliary propeller accelerates the rotor smoothly at low wind speed, while at high wind speed, the auxiliary propeller decelerates the rotor. Thus the auxiliary propeller works as a speed control system of the rotor.
A similar idea is described in WO 2007/045244 by the present applicant, where the circular root region and the transition region are replaced by two blade segments having a lift-generating airfoil profile.
CA 2 425 447 discloses a wind turbine blade unit comprising two blades disposed in a canard type configuration, i.e. a smaller airfoil arranged in front of a larger airfoil. This configuration provides a passive solution for self-adjusting the pitch angle of the wind turbine blade so as to control the wind turbine rotor hub rotation speed.
WO 02/055884 discloses a rotor for a water turbine or a water pump, the rotor comprising a number of vanes having a main vane blade and a secondary vane blade. The distance between the main vane blade and the secondary vane blade may be adjustable.
None of the cited prior art documents deals with the problem of improving the aerodynamic performance of the actual root region or the transition region of a wind turbine blade according to the afore-mentioned design and particularly not with regards to improving the lift-to-drag ratio of substantially circular, elliptical or oval profiles.
It is an object of the invention to obtain a new blade, which overcomes or ameliorates at least one of the disadvantages of the prior art or which provides a useful alternative.